1. FIELD OF INVENTION
This invention relates to an image processing method and apparatus, and, more particularly, to an image processing apparatus, for example, a laser printer, an LED printer, or the like, for forming images of a plurality of colors based on image signals transmitted from the host computer and so on, and, for example, a printer apparatus for controlling image data by storing it in an auxiliary disc storage device or the like.
2. THE PRIOR ART
A multi-color recording apparatus has until now been widely used as an output apparatus of a computer system. A typical laser beam printer shown in FIG. 21 comprises a printer engine 57 for recording an image by using a photosensitive drum (not shown) in accordance with dot data and a printer controller 56 arranged in a manner such that it receives code data supplied from an external host computer 52, generates page information consisting of dot data in accordance with the supplied code data, and successively transmits the dot data to printer engine 57. Host computer 52 selects and loads application software from a storage device, such as a floppy disc, which stores various kinds of application software, so as to start the program in the application software. Host computer 52 then performs a color image process by using the above-described application software to generate multi-color information, or to store the information.
FIG. 22 is a timing chart which illustrates a transferring sequence of signal processing circuits 58 through 62 of printer engine 57.
The dot data supplied from printer controller 56 are transferred to a color conversion processing circuit 58 in the form of color image signals, signals R (red), G (green) and B (blue). The transferred signals R, G and B are converted by color conversion processing circuit 58 into color image signals for printing, that is, signals M (magenta), C (cyan) and Y (yellow). A Bk (black) generating circuit 60 extracts a Bk signal from the above-described signals Y, M and C. A masking processing circuit 59 subjects the signals Y, M and C to a variety of signal processing including a masking process. Signals Y, M, C and Bk, which are the outputs from masking processing circuit 59 and Bk processing circuit 60 are stored in memory 61 so as to be sequentially transmitted to OR gate 63 by switching control circuit 62. Output 65 from OR gate 63 is subjected to a binarization processing such as dithering by binarization circuit 66 and is transmitted to a laser driver disposed in printing unit 67.
Image signals denoting characters to be formed by combinations of principle toner colors or the signals Y, M, C and Bk transferred from host computer 2 are received in the form of signals Bi. The signals Bi are supplied to 0R gate 63 so as to be synthesized with the signals Y, M, C and Bk which have been switched through switching control circuit 62. As a result, the synthesized signals are transmitted to the laser driver.
Recently, the use of page printers typified by a laser beam printer has rapidly increased because they have been superior to conventional dot matrix type serial printers in terms of image quality, output speed and silence in operation. A page printer of this type comprises an image forming controller for creating bit map image data (to be abbreviated to "image data" hereinafter) for one page from printing data for a text or an image transmitted from a host computer or the like, and a printer engine for exposing the image data transmitted from the image forming controller to a photosensitive body so as to print it in accordance with a sequence including the toner development, transfer to paper and fixing of the image.
In recent years, the printing data transmitted from the host computer has been mainly expressed by PDL a (Page Description Language) such as Post Script .RTM.. Therefore, the hardware of the image forming controller usually comprises a high speed microprocessor and an image memory (RAM) which stores image data of one page. For example, a binary (white and black) printer engine capable of printing an image on a printing paper sheet, size A4 (296 mm .times.210 mm) conforming to JIS (Japan Industrial Standard), at a dot density of 300 dpi (dots per inch), requires an image memory having a capacity of about 1 megabyte.
The capacity of the image memory inevitably increases excessively when the spatial resolution of the formed image is improved, multi-valued data is used, and a full color image is formed. In such a case, the size of the memory for the paper size is A4 is described below. A binary printer capable of printing an image at a dot density of 600 dpi requires a memory the capacity of which is about 4 megabytes, a multiple-value (8-bits, that is, 256 gradations or steps) printer requires a memory the capacity of which is about 8 megabytes, and a 300 dpi full color (each of color components Y, M, C and Bk represented as 8-bit data) printer requires memory the capacity of which is about 32 megabytes.
As shown in FIG. 22, if memory 61 for storing data for four color components is not provided, signals R, G and B for the same image must be supplied three times from host computer 52 or printer controller 56 so as to sequentially generate signals Y, M and C. As a consequence, host computer 52 cannot perform ensuing processes during the time in which signals R, G and B are being transmitted, causing the total throughput to be lowered. However, since host computer 52 must hold the signals R, G and B until three repetitions of the outputs of the signals R, G and B are completed, the load of host computer 52 becomes too great.
A large memory of about 24 megabytes is required in order to hold signals R, G and B using a semiconductor memory in a case where the printer paper size is A4, the spatial resolution is 300 dpi and the gradation is 256 steps for each color. As a consequence, the overall cost cannot be reduced.
Accordingly, in order to reduce the overall cost while using a large capacity memory, the following two countermeasures have been taken:
(1) Employ an external storage unit such as a hard disc, yielding a reduced cost and a large capacity for image memory; or PA1 (2) Reduce the memory size by making use of an image data compression technique. PA1 (1) Since transmission of an external image signal from external apparatus such as, for example, a host computer or a printer controller, and the process of forming an image by the image processing apparatus can be independently performed, the loads of the host computer and the printer controller can be reduced. PA1 (2) Since transmission of an external image signal, for example, a host computer or a printer controller, and the process of forming an image by an image processing apparatus can be independently performed, the degree of freedom in the transmission of the image signal can be increased. In consequence, both a serial signal and a parallel signal can be transmitted. PA1 (3) The image signal can be read from the storage device at a high speed.
However, when A4 paper sheets are printed at a speed of 8 sheets/minute at a spatial resolution of 300 dpi, image signals must be read at 1.5 megabyte/second so as to be stably supplied to the laser diode or the like in a case where an auxiliary storage unit such as a hard disc is used for the purpose of holding the signals R, G and B. Usually, Since the auxiliary storage unit is capable of reading data at a speed of about 700 to 800K byte, second data cannot be printed precisely at high speed if the auxiliary storage unit is used in accordance with the above-described conventional method.
If an external storage unit such as the hard disc is used as an image memory in place of a semiconductor memory such as a DRAM, the cost of the memory can be reduced by a factor of between 20 and 100. However, since hard discs or the photoelectromagnetic discs store data on each track (usually, one track is able to store data of 8K to 20K bytes),it takes 5 to 15 msec, which is very long, for the head to be moved to the track (to seek the track) from which desired data is to be read or to which desired data is to be written. Even worse, the data transferring speed within a track has been the unsatisfactory speed of 5 to 10 megabyte/sec. Therefore, a critical technical problem arises in using a hard disc or a photoelectromagnetic disc in place of a image memory of the page printer. Another problem arises when an external storage unit is used as the image memory in a manner such that a microprocessor in the image forming controller operates it; a virtual memory control is necessary. Therefore, the structure of the image forming controller becomes too complicated.
In a case where the size of the memory is reduced by employing an image compression technique, it takes an excessively long time to compress the image data as well as to reexpand the data read from the image memory so as to obtain image data of complete bit map when novel image data is generated by recursively utilizing the image data which has been temporarily stored in the image memory, as well as the image data is printed out with Post Script .RTM. like processing. When the compression and the expansion are performed by means of hardware, another problem arises in that a control circuit, the structure of which is quite complicated, must be used.